1. Field of the Invention
The field of the invention is a method and apparatus for extruding a parison (a tube of molten thermoplastic material) which is then placed in a die for blowmolding a container from the parison. More specifically, the present invention relates to a method and apparatus for varying the wall thickness of the parison and the swell--the outer and inner diameter--of the parison as the parison is extruded so that large containers with small neck entrance openings can be formed from the parison.
2. Description of the Prior Art
Heretofore it has been proposed to control the wall thickness of a parison by altering the amount of molten thermoplastic material which is extruded through a die head. In one such system disclosed in Kim et al. U.S. Pat. No. 4,029,458, a hydraulic or pneumatic activating system is utilized for raising or lowering a mandril which is situated completely internally of the die head to control the wall thickness of the parison which is extruded from the die head. However, with the mandril located within the die head, movement of the mandril, although controlling the wall thickness of the parison, does not affect of significantly alter the effective outer diameter of the parison extruded from the die head.
Also it has been proposed in the Stevens U.S. Pat. Nos. 3,221,371 and 3,312,766 to vary the wall thickness of a parison being extruded by adding material to the parison as it is being extruded. More specifically, these patents disclose that the wall thickness of the parison can be altered by adding material to the outside circumference of the parison thereby increasing the outer diameter of the parison without however altering the inner diameter of the parison. Also these patents teach the varying of the wall thickness of the parison by controlling the flow of molten thermoplastic material out of a fixed annular orifice located on the inside or the outside of the primary die annular orifice. Also, both the primary and secondary annular orifices open onto the bottom of the die head.
In the conventional blowmolding of a container, an extruded parison is circumscribed by a female mold. Then, air is blown internally into the tube or parison to expand the size thereof against the inner walls of the mold so that the parison takes the shape of the mold to form a container. It will be appreciated that the larger the ratio of the diameter of the mold to the outside diameter of the parison the harder it is to uniformly distribute the plastic material.
When blowmolding a large container with a small opening, the outside diameter of the parison cannot exceed the diameter of the small opening. The size of the small opening then limits the size of the parison and the blowup ratio, i.e., the ratio of the outer diameter of the parison to the inner diameter of the mold which is preferably not greater than 1:4.
If one could vary the swell of the parison, that is, the ratio of the inner diameter of the parison to the outer diameter of the ring shaped annular outlet orifice from which the parison is extruded, then a large container with a small neck entrance opening can be formed. Presently the only known method for obtaining a large container with a small entrance opening is by preblowing the parison. This is done by the injection of air internally of the parison before the mold closes so as to form a bladder with a small neck. Unfortunately, however, there is still a problem of varying the wall thickness of the parison since the part of the parison that will be expanded the most to fill the mold and form the container has to have a sufficient wall thickness of material.
It will be appreciated that if the wall thickness of the parison is barely sufficient to expand to a desired wall thickness for the container in the mold, the weakest portion of the parison will expand first getting thinner than the rest of the parison. In this respect the thinner sections expand first contacting the mold and offsetting the parison towards the opposite wall. This problem becomes more pronounced as the blowup ratio (outer diameter of the parison to the inner diameter of the mold) increases.
As will be described in greater detail hereinafter, the method and apparatus for extruding a parison of the present invention differ from the previous methods and apparatus by providing for the varying of the wall thickness and the varying of the swell of the parison as it is being extruded to produce a parison which has sufficient wall thickness where the greatest amount of expansion will take place and which has a larger swell so as to reduce the blowup ratio thereby to provide a parison from which a large container with a small neck entrance opening at the end of the parison where minimum wall thickness and minimum swell are provided can be formed from the parison.